The present invention relates to an electron-gun system for multi-cathode-ray tubes, such as color picture tubes. More specifically, the invntion pertains to a system comprising cathodes and several electrodes following them and lying behind each other in the electron-beam direction, of which at least one electrode has a substantially greater spatial extension in the electron beam direction than the other electrodes, and comprises at least two segments, with the electrodes being made of different materials.
One such electron-gun system is known from German OS No. 2,920,151.
In this conventional electron-gun system, at least the first three electrodes when looked at in the beam direction, are made of different materials. The temperature expansion coefficients of the materials of the electrodes are staggered increasingly from the cathode to the screen in such a way that the distance variations in the beam direction between the apertures of the electrodes lying next to each other, and through which the electron beams are permitted to pass, decrease linearly when the electron-gun system is at operating temperature.
By taking this measure the voltages occurring in the electron-gun system due to it being heated up to high temperatures (ranging between 91.degree. to 315.degree. C.), remain as low as possible. Furthermore, the electron-optical lenses existing between the electrodes can have an undisturbed effect upon the electron beams.
These conventional types of electron-gun systems have proved well in practice. The pictures produced by the electron beams in a color picture tube, however, should not only be convergent in the operating state of the color picture tube but also shortly after the color picture tube is put into operation. In a color picture tube, the warming-up time may last several minutes. It has been observed that during this warming-up time of the color picture tube there appear very noticeable misconvergencies.
These misconvergenices are due to the fact that during the warming-up time of the color picture tube, because of different expansion velocities of the materials, there appears a temporarily occurring displacement between the electrodes G3 and G4. This leads to a distortion of the electron-optical lens between G3 and G4.
It was found that in the case of an "inline" electron-gun system comprising "unitized guns", in which the corresponding electrodes of all three electron beams are united in one body, this displacement between one electrode aperture in the electrode G3 of an electron beam lying outside, and the corresponding electrode aperture in the electrode G4 in the case of a center spacing of the electrode apertures of 6.6 mm, amounts to about 1.5 .mu.m. This causes the displacement of the phosphor dot energized by an electron beam passing through these apertures, in the center of the screen of a 27" tube, with respect to the neighboring phosphor dot by about 0.2 mm in the case of a voltage of 18 kV between the electrodes G3 and G4. The displacement of the phosphor dots as produced by the two outer electron beams of a color picture tube in relation to one another, that is of the red and the blue phosphor dots, will then amount to about 0.4 mm in the center of the screen. This is a clearly visible misconvergence.
Any possibly remaining residual misconvergence in the steady-state condition of the color picture tube, that is, when the electron-gun system thereof has reached its operating temperature can be corrected in the conventional way with the aid of a convergence unit. However, a color picture tube may take up to thirty minutes to reach its operating temperature and this misconvergence during the warming-up period is undesireable.